Traditionally flexible pipe is utilised to transport production fluids from one location to another.
Production fluids are fluids such as oil and/or gas and/or water when conveyed (at least in part) in a sub-sea environment, such as in connection with oil extraction.
Flexible pipe is particularly useful in connecting a sub-sea location to a further sub-sea location or sea level location. Flexible pipe is generally formed as an assembly of flexible pipe body and one or more end fittings. The pipe body is typically formed as an assembly of layered materials that form a fluid and pressure-containing conduit. The pipe structure allows large deflections without causing bending stresses that impair the pipe's functionality over its lifetime. The pipe body is generally, but not necessarily, built up as a composite structure including unbonded metallic and polymer layers.
Such unbonded flexible pipe has been an enabler for deep water (less than 3300 feet (1006 meters)) and ultra-deep water (greater than 3300 feet) developments for over 15 years. Available technology enabled the industry to initially produce in deep water in the early 1990s and in ultra-deep waters up to around 6500 feet (1981 meters) in the late 1990s. Water depths greater than 6500 feet push the envelope where typical free hanging riser configurations and flexible pipe in general can operate.
It is highly desirable to maintain the fluid in the bore of a flexible pipe at or above a suitable temperature, for example above 0° C., more especially about 40° C. or more, to prevent the formation of solids in the fluid, of which waxes and hydrates are particularly significant examples. Such solids tend to form when the temperature of the production fluid falls below a certain (known) temperature and thus cause blockages in the pipe. The temperature at which solids begin to form will vary depending on, for example, the particular composition of the fluid, but a value of about 40° C. is common. Specific means for maintaining the desired temperature can be desirable during normal operation but may not be necessary since the temperature of the fluid being conveyed is often much higher than the temperature at which solids begin to form in the fluid. Maintaining a desired minimum temperature is particularly important at times when the fluid is subject to significant cooling, notably also during periods of shut down when the fluid is essentially static within the pipe.
Conventionally, the art has sought to achieve this minimum temperature through insulation layers that form part of the pipe structure and which are intended to reduce the rate of heat loss from the bore fluid into the external environment. For flexible pipes, these insulating layers are positioned between the pressure sheath and the outer sheath of the pipe, most commonly between an outer armour layer and the outer sheath.
One prior art method of passive insulation seeks to limit the heat lost to the environment by providing insulation between the carcass and pressure sheath, thereby to isolate bore fluid from the external environment and enable heat retention. This method is disadvantageous in the sense that it simply reduces the rate at which heat is lost from the fluid contained in the bore rather than actively reintroducing heat to maintain the bore temperature.
Another prior art method consists of actively heating a flexible pipe with electrical energy, hot water, steam or other direct internal heating of the pipe. This is advantageous in the sense that it can be applied indefinitely within the pipe system. However a very significant disadvantage is that a significant amount of power is required and it is a more complicated system than passive insulation.
Occasionally, as an alternative, a cold fluid is transported along the bore of the flexible pipe. Again it is helpful to regulate the temperature of such fluids to prevent undesired heating of the fluid from a surrounding relatively warm environment.
A known prior art technique for providing a thermal insulation layer is to wind tape manufactured from a thermally insulating material helically around an underlying layer during manufacturing of flexible pipe body. Sometimes a tape formed from a polypropylene matrix with hollow glass spheres has been used which provides a low thermal conductivity (k) value and which is able to withstand reasonably high hydrostatic pressures. However, the hollow glass spheres in the tape are prone to crushing and internal and external pressures operate to squeeze the tape layer thereby reducing thickness and thus thermal insulation effects.
A further prior art technique for providing a thermal insulation layer is to extrude a layer of thermally insulating material over an underlying layer during manufacturing of flexible pipe body. Commonly a polymer matrix containing air bubbles and/or glass spheres has been used. However, this includes an additional and complicated manufacturing step which can be a complex process involving careful alignment, heating and cooling steps during manufacture. Also geometric tolerances are difficult to control during an extrusion process and this can have a knock on effect to subsequent layers formed radially outside such an extruded layer.
With either the tape or extruded layer technique mentioned above the materials used have until now limited the thermal conductivity (k) value available.
Further, the insulating materials used to date have limited heat storage capacity and have not proved able to prevent the solidification of waxes, hydrates and like materials in the pipe for a desired length of time, especially during periods of shut down.